builder: add loop helpers (create_loop_blocks/add_predecessor) and adopt in LoopBuilder; use BinaryExpr/CallExpr wrappers in expr lowering (no behavior change)

src/mir/builder/loops.rs

@@ -1,5 +1,14 @@
 //! Small loop utilities for MirBuilder
-use super::BasicBlockId;
+use super::{BasicBlockId, MirBuilder};
+
+/// Create header/body/after blocks for a loop and push loop context.
+pub(crate) fn create_loop_blocks(builder: &mut MirBuilder) -> (BasicBlockId, BasicBlockId, BasicBlockId) {
+    let header = builder.block_gen.next();
+    let body = builder.block_gen.next();
+    let after = builder.block_gen.next();
+    push_loop_context(builder, header, after);
+    (header, body, after)
+}
 
 /// Push loop context (header/exit) onto the MirBuilder stacks.
 pub(crate) fn push_loop_context(
@@ -36,3 +45,19 @@ pub(crate) fn in_loop(builder: &super::MirBuilder) -> bool {
 pub(crate) fn depth(builder: &super::MirBuilder) -> usize {
     builder.loop_header_stack.len()
 }
+
+/// Add predecessor edge metadata to a basic block.
+pub(crate) fn add_predecessor(
+    builder: &mut MirBuilder,
+    block: BasicBlockId,
+    pred: BasicBlockId,
+) -> Result<(), String> {
+    if let Some(ref mut function) = builder.current_function {
+        if let Some(bb) = function.get_block_mut(block) {
+            bb.add_predecessor(pred);
+            return Ok(());
+        }
+        return Err(format!("Block {} not found", block));
+    }
+    Err("No current function".to_string())
+}